Field of the Invention
The present invention relates to an image processing apparatus and an image processing method used for ophthalmological consultations.
Description of the Related Art
Ophthalmic examinations are widely performed for the purpose of early diagnosis of lifestyle-related diseases and diseases that rank highly among causes of loss of eyesight. A scanning laser ophthalmoscope (SLO), which is an image processing apparatus that uses the principle of a confocal laser microscope, is an apparatus that performs Raster scanning of an eye fundus using a laser as a measuring beam and obtains a planar image at a high resolution and a high speed based on the intensity of the return light. The apparatus that captures this planar image will be referred to as an SLO apparatus, and the planar image will be referred to as an SLO image below.
In recent years, it has been possible to obtain a retinal SLO image with an improved horizontal resolution by increasing the diameter of the measuring beam in the SLO apparatus. However, there has been a problem in acquiring a retinal SLO image in that increasing the diameter of the measuring beam is accompanied by a decrease in the S/N ratio and the resolution of the SLO image due to aberrations in the eye of the examination subject.
In order to resolve the above-mentioned problem, an adaptive optics SLO apparatus has been developed that has an adaptive optics system that measures aberrations in the eye of the examination subject in real-time using a wavefront sensor and corrects aberrations of a measuring beam or its return light that occur in the examination subject eye using a wavefront compensation device, thereby enabling the acquisition of an SLO image with a high horizontal resolution.
This SLO image having a high horizontal resolution can be obtained as a moving image, and in order to observe blood flow dynamics for example in a non-invasive manner, retinal blood vessels are extracted from the frames of the moving image, and the movement speed and the like of blood cells in capillaries are subsequently measured. Also, in order to evaluate the relationship between the photoreceptor cells and the visual function using the SLO image, photoreceptor cells P are detected, and subsequently the density distribution and the alignment of the photoreceptor cells P are measured. FIG. 6B shows an example of an SLO image with a high horizontal resolution. The photoreceptor cells P, a low luminance region Q that corresponds to the position of a capillary, and a high-luminance region W that corresponds to the position of a leukocyte can be observed.
In the case of observing the photoreceptor cells P, measuring the distribution of photoreceptor cells P, or the like using the SLO image, the focus position is set near the outer layer of the retina (B5 in FIG. 6A) and an SLO image such as FIG. 6B is captured. On the other hand, there are retinal blood vessels and bifurcated capillaries in the inner layers of the retina (B2 to B4 in FIG. 6A). FIG. 6A shows an example of the various layers in the retina, from the inner limiting layer B1 to a pigmented layer B6. 45% of the blood that exists in blood vessels is composed of blood cell components, and of those blood cell components, about 96% are erythrocytes and about 3% are leukocytes. An erythrocyte has a diameter of about 8 μm, and a neutrophil, which is the most common type of leukocyte, is 12 to 15 μm in size.
With lifestyle-related diseases and systemic diseases such as diabetes, it is known that symptoms appear which indicate a decrease in blood fluidity (the extent to which blood flows smoothly). Specific examples of this include a decrease in blood cell deformability, and erythrocytes and thrombocytes tending to aggregate. In blood vessels, erythrocytes are constantly aggregated here and there as shown in FIG. 6C (which also shows flow direction FD), and erythrocyte aggregates DTi are formed. If the focus position is set to the photoreceptor cells and an SLO image having a high horizontal resolution is obtained, shadows will form in the vicinity of the photoreceptor cells since incident light does not pass through the erythrocytes, and an erythrocyte aggregate DTi is rendered as a dark tail DTi. On the other hand, if the blood returns to the normal state, for example, due to medical treatment, it is conceivable that the number of erythrocyte aggregates that are constantly aggregated as shown in FIG. 6D will gradually decrease. Conventionally, the number of erythrocyte aggregates flowing in blood vessels and their change over time could not be measured in a non-invasive manner.
A conventional technique of generating a spatiotemporal image for a capillary branch region in an adaptive optics SLO moving image and of measuring the degree of physiological erythrocyte aggregation based on the change in the length of an erythrocyte aggregate in the spatiotemporal image is disclosed in “Uji, Akihito, ‘Observation of dark tail in diabetic retinopathy using adaptive optical scanning laser ophthalmoscope’, Proceedings of the 66th Annual Congress of Japan Clinical Ophthalmology, p.27 (2012)” as a technique for measuring blood fluidity in a non-invasive manner.
However, in the above-described technique, the degree of erythrocyte aggregation that (temporarily) appears physiologically is measured based on the change in the length of the erythrocyte aggregate, and no technique of measuring the distribution (number according to region) of (constant) erythrocyte aggregates caused by abnormal erythrocyte aggregation that appears here and there in the blood vessel is disclosed.